Pixel density and dot pitch.

Worked example

A 27 inch screen at 2560 x 1440 has a diagonal of sqrt(2560^2 + 1440^2) = 2937.2 pixels, so PPI = 2937.2 / 27 = 108.79 PPI. The pixel pitch is 25.4 / 108.79 = 0.233 mm. The active area is 23.53 in x 13.24 in. One pixel (1 / 108.79 in) subtends 1 arcminute at about 31.6 in.

How it works

As the Pixel density - Wikipedia article describes, PPI is calculated in two steps: the diagonal in pixels from the Pythagorean theorem, then PPI = diagonal pixels / diagonal inches. Pixel pitch is the inverse, 25.4 mm / PPI. The Dot pitch - Wikipedia article describes dot pitch as the distance between dots on a display, measured in millimetres (smaller means higher resolution) or as a rate such as dots per inch.

The blend distance is 1 / PPI divided by the tangent of 1 arcminute, the usual 20/20 acuity figure. It is geometry, not a quality rating. Pixel pitch here is the pixel-to-pixel spacing; the sub-pixel dot pitch of an RGB panel differs.

Sources: Pixel density - Wikipedia; Dot pitch - Wikipedia. Each opened 2026-10-08.

Planning guide only, not professional advice. Figures are estimates from the inputs you enter.

FAQ

How do I calculate PPI?

Take the diagonal resolution in pixels, sqrt(width^2 + height^2), and divide it by the screen diagonal in inches. A 27 inch 2560 x 1440 screen is 108.8 PPI.

How do I convert PPI to dot pitch?

Divide 25.4 mm by the PPI. At 108.8 PPI the pixel pitch is 0.233 mm.

Does a higher PPI always look sharper?

It makes pixels smaller, but what you can see depends on viewing distance. Beyond the blend distance (1 arcminute per pixel) a 20/20 eye cannot separate adjacent pixels.

Is PPI the same as DPI?

No. The Wikipedia pixel density article treats PPI (pixels per inch) and printer DPI (dots per inch) as different measures, because a pixel is a single sample of any colour while a printed dot is on or off.

Does the aspect ratio matter?

Not directly. Only the pixel counts and the diagonal enter the calculation.